A turbocharged Diesel engine system generally comprises a Diesel engine having an intake manifold and an exhaust manifold, an intake line for conveying fresh air from the environment in the intake manifold, an exhaust line for conveying the exhaust gas from the exhaust manifold to the environment, and a turbocharger which comprises a compressor located in the intake line for compressing the air stream flowing therein, and a turbine located in the exhaust line for driving said compressor.
The turbocharged Diesel engine system further comprises an intercooler, also indicated as Charge Air Cooler (CAC), which is located in the intake line downstream the compressor, for cooling the air stream before it reaches the intake manifold. The turbocharged Diesel engine systems can also be equipped with a diesel oxidation catalyst (DOC) for degrading residual hydrocarbons and carbon oxides contained in the exhaust gas and, downstream of the DOC, a diesel particulate filter (DPF) for capturing and removing diesel particulate matter (soot) from the exhaust gas.
In order to reduce the polluting emission, most turbocharged Diesel engine system actually comprises a first exhaust gas recirculation (EGR) system, for selectively routing back exhaust gas from the exhaust manifold into the intake manifold. In such a way the exhaust gas mixed with the fresh induction air is aspired into the engine cylinders, in order to reduce the production of unburned hydrocarbon (HC), carbon monoxide (CO), soot, and oxides of nitrogen (NOx) during the combustion process. In order to further reduce the NOx emission, improved EGR systems comprise an additional EGR conduit, which fluidly connects the exhaust line downstream the DPF with the intake line upstream the compressor of turbocharger, an additional EGR cooler located in the additional EGR conduit, and additional valve means for regulating the flow rate of exhaust gas through the additional EGR conduit.
In these improved systems, while the conventional EGR conduit defines a short route for the exhaust gas recirculation, the additional EGR conduit defines a long route for the exhaust gas recirculation, which comprises also a relevant portion of the exhaust line and a relevant portion of the intake line. Flowing along the long route, the exhaust gas is then obliged to pass through the turbine of turbocharger, the DOC, the DPF, the additional EGR cooler, the compressor of turbocharger and the charge air cooler, so that it become considerably colder than the exhaust gas which flows through the short route, reaching thereby the intake manifold at a lower temperature.
These improved EGR systems are generally configured for routing back the exhaust gas partially through the short route and partially through the long route, in order to maintain the temperature of the induction air in the intake manifold at an optimal intermediate value in any engine operating condition.
In the known art the total amount of exhaust gas and the long route exhaust gas rate are determined by the Electronic Control Unit (ECU) using empirically determined data sets or maps, which respectively correlate the total amount of exhaust gas and the long route exhaust gas rate to a plurality of engine operating parameters, such as for example engine speed, engine load and engine coolant temperature. One drawback of these improved EGR systems is that such data sets or maps are determined during a calibration activity, using an engine system perfectly efficient which is operated under standard environmental conditions, i.e., standard environmental temperature, pressure and moisture. Therefore, the value contained in the data sets or maps are valid only for engine systems that are operated in the same environmental conditions of that used in calibration phase, and completely ignore the reduction in efficiency of the engine system components due to several conditions that may occur during use of the vehicle.
For example, it has been observed that in some real use conditions of the vehicle, such as for example high-altitude and/or high-temperature operation and repeated accelerations a series of problems may occur. For example various components may drift from their expected operation parameters leading to sub-optimal control of the engine by the ECU or even components damage. Furthermore, long-route EGR cooler fouling may occur and temperatures out of specifications may be reached downstream of compressor and in the engine intake manifold. It is clear that these problems would lead to components damage due to thermal stress and/or excessive oil cracking and deposition, or at least to a reduced life of engine components with an associated increase of costs.
Due to this situation, the known art has tried to solve the above problems by ensuring protection against excessive temperatures downstream of compressor as well as over-speed are performed in open-loop, with the help of undesirable significant engineering margins. In case of the presence of a long-route EGR system this disadvantage increases, since at mid-load, in the EUDC area, compressor protection is enacted in open loop too, severely limiting the system performance. It appears therefore that these solutions are unsatisfactory and may even be considered palliative.
At least one object is to create a device and a method that allows protecting the compressor and downstream pipes from thermal stress, from oil cracking, and allowing operating the compressor with reduced engineering margin with respect to the current situation. At least another object is to provide such protection strategy taking advantage from the computational capabilities of the Electronic Control Unit (ECU) of the vehicle. At least a further object is to meet these goals by means of a simple, rational and inexpensive solution. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.